1. Field of the Invention
The present invention relates to a glass antenna system for an automobile in which the impedance of a glass antenna provided on the automobile is matched for that of a car radio or receiver connected through a transmission cable. It further relates to a glass antenna system comprising FM and AM antennas each provided on the window glass of the automobile, FM and AM dynamic matching circuits each connected to the corresponding antenna and a common transmission cable.
2. Description of the Prior Art
The inventors proposed with Japanese patent application laid open No. 3-49402 (1991) and utility model application No. 63-143591 (1988) a glass antenna system for an automobile without any preamplifier. The glass antenna 2 as shown schematically in FIG. 1 is connected to a conventional dynamic matching circuit 3 provided on a rear glass 1 and having its circuit factor corresponding to the selected frequency of the receiver 6. On the rear glass 1, a plurality of heaters or wires 4 is printed as an AM antenna 4, while the FM antenna 2 is printed on the upper portion of the most significant heater 4. When the FM antenna 2 is printed on the rear glass 1, its size or dimension is limited even if patterns of the antenna are designed. Accordingly, the resistance component of the FM antenna 2 is often changed between the higher and lower values than that of a transmission cable 5.
FIG. 2 generally shows the active matching circuit 3 comprising a capacitor 10 connected between an input terminal 11 and an input node 12, and a coil 13 connected between the input node 12 and an output node or terminal 14. The input terminal 11 is also connected to the FM antenna 2. The matching circuit 3 further comprises a DC cutoff capacitor 15 having relatively high capacitance (e.g. 0.1 nF) and connected between the input node 12 and a relay node 16, and a varactor diode 17 connected between the relay node 16 and output node 14. A resistor 18 and capacitor 19 each connected to ground are connected to the relay node 16 and the terminal 14 respectively.
The anode of the varactor 17 is grounded through the resistor 18. Therefore, the coil 13, capacitor 15 and varactor 17 constitute a resonance circuit tuned to a predetermined frequency, to control the resonance frequency by a voltage SE applying to a cathode of the varactor 17 through the cable 5.
FIG. 3 shows a basic principle circuit of the conventional dynamic matching circuit shown in FIG. 2. As apparent from an impedance matching theory, the circuit shown in FIG. 3 does not have a matching function when the resistance component Rx of the antenna impedance is higher than that RL of the load impedance. The conventional matching circuit cannot be applied to such glass antenna patterns in case of the Rx more than the RL. The impedance of the load side must have only the resistance component.
When the characteristic impedance of the cable 5 is different from the input impedance of the receiver 6, the impedance viewed from an end of the cable 5, that is, the matching circuit to the receiver includes a marginal or reactance component. The gain of the receiving system is reduced because of mismatching between the impedance of the antenna and that of the receiver 6 connected through the cable 5.
As described above, the conventional receiving system having the dynamic matching circuit connected to the glass antenna must fulfill the condition that the resistance component of the antenna impedance is smaller than that of the load impedance of the receiver 6 viewed through the cable 5. Because the antenna impedance range being available is limited and its size of the antenna pattern is then limited, adjustments of its directivity and gain are also limited. Therefore, it is impossible to obtain a suitable glass antenna pattern having desired gain and directivity.
The conventional receiving system has reduced receiving system gains because the impedance of the glass antenna side does not adequately match that of the load side of the receiver in case that the characteristic impedance of the cable is different from the input impedance of the receiver.
FIG. 4 shows another prior art of the antenna system having preamplifiers 7 and 8. The same numerals are denoted in parts corresponding to the those of FIG. 1. The FM preamplifier 7 is connected between the FM antenna 2 and the cable 5 while the AM preamplifier 8 is connected between the AM antenna 4 and the cable 5.
In such a conventional receiving system, the FM preamplifier 7 is active when an AM signal is received by the receiver 6. The FM preamplifier 7 has a wide frequency characteristic over the FM broadcast band and does not have a sharp selectivity against a predetermined electric wave to be desired. When the automobile is moved to the area in which strong electric fields of radio waves radiated by at least two FM broadcast stations are overlapped and their frequency's difference is entered within a certain frequency of the AM frequency band, by the intermodulations among those FM radio waves in the FM preamplifier 7 and the receiver 6, undesirable AM signals are generated.
Accordingly, in the conventional receiving system for the automobile having preamplifiers, when the automobile is moved to the area that electric fields of radio waves issued by two or more of FM broadcast stations are strong and their frequency's difference is entered within the AM frequency band, intermodulation noises by those FM radio waves are generated upon receiving the electric wave from the AM station.